The present invention relates to a downhole pump for producing oil and gas wells, and more particularly to a modular downhole multiphase pump.
Downhole pumps are used to provide artificial lift in order to produce petroleum products from oil and gas wells. Conventional downhole pumps include pumpjack type pumps, progressive cavity pumps (xe2x80x9cPC pumpsxe2x80x9d) and electric submersible pumps (or xe2x80x9cES pumpsxe2x80x9d).
ES pumps are downhole centrifugal pumps to which power is supplied by a cable running though the well bore from the surface. These pumps are well known and widely used but suffer from comparatively low pumping efficiency and an inability to handle solids entrained in the production fluid.
PC pumps are well known and use a metal rotor rotating within an elastomeric stator to lift production fluids to the surface through the well""s tubing string. PC pumps may be powered from a surface top drive through a sucker rod or by electric submersible motors located below the pump at the end of the tubing string. While an improvement over walking beam pumps, conventional PC pumps present many disadvantages. The abrasive downhole environment quickly degrades the elastomeric stator. Aromatic compounds may cause the stator to blister and degrade. The frictional contact between the rotor and stator also wears out both the rotor and the stator. Replacing or repairing a downhole PC pump involves expensive downtime for the well because the entire tubing string must be pulled up by a service rig.
As well, PC pumps are most efficient if they are pumping liquids only. They do not function well with multiphase operations with high gas content. Very often, gases and solids are produced with liquids and may cause rapid degradation and wear in the PC pump.
Another significant problem with PC pumps is the potential for backspinning caused by reverse flow of fluid through the pump. This situation may occur if an electrically powered top drive unit suffers a power failure. A typical well may have a PC pump which is hundreds of meters underground. The column of production fluid within the tubing string will fall back through the PC pump, causing the pump, the sucker rod and associated surface machinery to backspin at very high rates. In some cases, the backspinning may cause components in the top drive to fail catastrophically, with the potential to injure personnel in the vicinity of the top drive.
Therefore, there is a need in the art for a downhole pump which migrates the disadvantages of the prior art above.
In general terms, the invention comprises a modular pump for underground use in connection with an oil well having a drive source which may be a surface drive which rotates a sucker rod or which may be a downhole motor. In one aspect of the invention, the pump comprises:
a) a rotor module comprising:
i) a cylindrical housing having a bottom face, a top face, a cylindrical face, an inlet, an outlet and an internal rotor enclosure between said inlet and outlet;
ii) at least one pair of intertwined and counter-rotating rotors within the internal rotor enclosure;
iii) means for rotating the rotors; and
b) a gear module having means for connecting to the drive source and means for outputting power to the rotor module.
The rotor module preferably comprises an upper and a lower pair of rotors separated by an intake plenum wherein the upper pair of rotors drives fluid upwards into an outlet plenum and the lower pair of rotors drives fluid downwards into a fluid passage which then joins the outlet plenum. The rotor module housing inlet preferably comprises an intake opening in the cylindrical face of the housing, which is in fluid communication with the intake plenum. Alternatively, the rotor module housing inlet comprises an opening defined in the bottom face of the housing and a inlet passage connecting the opening with the intake plenum.
In a preferred embodiment, the pump comprises at least one rotor module housing (hereinafter referred to as a xe2x80x9cvolume modulexe2x80x9d) having an inlet comprising an intake opening around the circumference of the housing, which is in fluid communication with the intake plenum; and at least one rotor module housing (hereinafter referred to as a xe2x80x9cpressure modulexe2x80x9d) having an inlet comprising an opening defined in the bottom face of the housing and a inlet passage connecting the opening with the intake plenum, wherein the outlet of the volume module connects with the inlet of the pressure module. More preferably, the pump further comprises a plurality of volume and pressure modules connected in series wherein the pressure modules are arranged in ascending order of pressure capacity.
The gear module may be adapted for use with a sucker rod as a power source, in which case it preferably is a speed increaser, or the gear module may be adapted for use with a downhole electrical motor as a power source. In addition, gear modules may be provided to transfer power from one rotor module to another.
In another aspect of the invention, the invention comprises a rotor module for use in assembling a modular pump for underground use in connection with an oil well, said rotor module comprising a cylindrical housing having an upper surface, a circumferential surface, a bottom surface and defining an internal rotor enclosure, said rotor module characterized in that:
(a) the bottom surface defines an fluid intake opening and a bypass opening;
(b) the circumferential surface defines a lateral intake opening; and
(c) the upper surface defines a bypass outlet opening and a fluid outlet opening;
wherein each of the openings communicates with the rotor enclosure and some of the openings may be plugged while others are kept open to control the flowpath of fluid into and out of the housing such that two or more rotor modules may be connected in series to increase the pressure and/or volume capacity of the pump.
In another aspect of the invention, the invention comprises a modular pump for underground use comprising at least one volume rotor module and at least one pressure rotor module connected in series wherein:
(a) the volume rotor module comprises a housing defining a rotor enclosure including an intake plenum, a flow inlet opening communicating with the intake plenum, interleaved and counter-rotating rotors within the rotor enclosure, and a flow outlet opening; and
(b) the pressure rotor module comprises a housing defining a rotor enclosure including an intake plenum, a single inlet opening which communicates with the flow outlet opening of the volume rotor module and with the intake plenum, interleaved and counter-rotating rotors within the rotor enclosure, and a flow outlet opening; and
(c) means for rotating the rotors in each of the volume rotor module and the pressure rotor module.
In the preferred embodiment, the modular pump comprises a plurality of volume rotor modules and a plurality of pressure rotor modules connected in series and in each such module, the intake plenum is a substantially central portion of the rotor enclosure and the rotors comprise an upper and a lower rotor pair which pump fluid in opposite directions such that each module is hydraulically balanced.